1. Field of the Invention
This invention relates to a hydraulic brake system for a vehicle which is able to perform anti-lock control and traction control (that, when a vehicle starts to move or increases speed, suppresses the spin of the wheels by application of braking forces to them), and more particularly to a hydraulic brake system for a vehicle in which, with the aid of a control valve provided in a passage through which a wheel brake adapted to apply a braking force to a driving wheel of the vehicle is hydraulically connected to a pressure generating chamber of a master cylinder, the wheel brake is hydraulically isolated from the pressure generating chamber and hydraulically connected to a low back pressure reservoir, so that the brake fluid in the wheel brake is caused to flow into the low back pressure reservoir thereby to decrease the hydraulic pressure in the wheel brake, and with the aid of the control valve, the wheel brake is hydraulically isolated from the pressure generating chamber and the low back pressure reservoir, and with the aid of a pump driven by an electric motor, the brake fluid in the low back pressure reservoir is caused to flow into the wheel brake, thereby to reincrease the hydraulic pressure in the wheel brake.
2. Description of the Related Art
A hydraulic brake system of this type is known in the art, being disclosed, for instance, by Japanese Patent Unexamined Publication No. Sho 62-134361.
The hydraulic brake system disclosed by Japanese Patent Unexamined Publication No. Sho 62-134361 has a bypass passage having a check valve which operates as follows: That is, in order that, when the hydraulic pressure in the pressure generating chamber of the master cylinder decreases, the hydraulic pressure in the wheel brake is decreased to the hydraulic pressure in the pressure generating chamber without fail, the check valve operates to hydraulically connect a wheel-brake-side passage through which the wheel brake is hydraulically connected to the control valve to a master-cylinder-side passage through which the pressure generating chamber is hydraulically connected to the control valve. And the check valve permits only the flow of brake fluid from the wheel brake to the pressure generating chamber. On the other hand, sometimes it is required to employ a hydraulic brake system for a vehicle which is able to perform both anti-lock control and traction control. The hydraulic brake system disclosed by Japanese Patent Unexamined Publication No. Sho 62-134361 cannot meet this requirement, being unable to perform the traction control.
The hydraulic brake system disclosed by Japanese Patent Unexamined Publication No. Sho 62-134361 may be made to be able to perform traction control and reduced in manufacturing cost by modifying it for instance as follows: A change-over valve is provided in the master-cylinder-side passage which is adapted to hydraulically isolate the control valve and the bypass passage from the pressure generating chamber and to hydraulically connect the inlet passage of the pump to the pressure generating chamber, or a cut-off valve is provided in the master-cylinder-side passage which is adapted to hydraulically isolate the control valve and the bypass passage from the pressure generating chamber, and another cut-off valve is provided which is adapted to hydraulically connect the inlet passage of the pump directly to a master cylinder reservoir provided for the master cylinder. However, this modification is suggested by Japanese Patent Unexamined Publication No's Hei 1-74153 and Hei 2-18150.
On the other hand, the modification of the system in the above-described manner gives rise to the following difficulty: When the added valves become out of order, the control valve and the bypass passage may be fixedly maintained hydraulically isolated from the pressure generating chamber. In this case, it may be impossible to supply brake fluid to the wheel brake or to draw brake fluid from the latter even if the brake pedal is operated (depressed or released).